1. Field of the Invention
The present invention relates to an image forming apparatus and a sheet conveying method, and in particular to an image forming apparatus and a sheet conveying method that correct for skew of a sheet being conveyed to an image forming unit.
2. Description of the Related Art
Image forming apparatuses such as copiers, printers, and facsimiles have a conveying unit that conveys a sheet such as a recording sheet or an OHP sheet to an image forming unit. According to those image forming apparatuses, a sheet is conveyed to the image forming unit by the conveying unit, and an image formed by the image forming unit is transferred to the sheet, so that an image is formed on the sheet.
Some of such image forming apparatuses having a conveying unit have a skew correction unit to correct for sheet skew so as to prevent a skewed sheet from being conveyed to the image forming unit, and correction systems using the skew correction unit include a loop forming system and an active registration system.
In the loop forming system, skew is corrected for by pressing a sheet onto a roller pair being at a standstill to form a loop in the sheet.
On the other hand, in the active registration system, skew is corrected for by turning a sheet while conveying it using two sensors and two roller pairs which are individually rotating (see Japanese Laid-Open Patent Publication (Kokai) No. H10-032682). Specifically, first, a skew amount of a sheet is computed based on a difference between generation timings of detection signals which are generated when a leading end of the sheet passes the two sensors provided on a common axis perpendicular to a sheet conveying direction of a sheet conveying path. Then, according to the computed skew amount, the rotational speeds of the respective two roller pairs provided at different locations in a direction perpendicular to the sheet conveying direction are individually controlled. For example, the rotational speed of one roller pair is controlled to be higher or lower than that of the other roller pair, so that skew of the sheet is corrected for.
Incidentally, to improve the performance of the image forming apparatuses, an image forming operation must be speeded up, but the above described loop forming system is not suitable because a sheet being conveyed must be temporarily stopped so as to form a loop in the sheet, and hence correction requires long time.
On the other hand, in the active registration system, skew of a sheet is corrected for without temporarily stopping conveyance of the sheet, and hence the time required for correction is shorter than in the loop forming system. Moreover, the sheet interval between a preceding sheet and a following sheet can be shortened as compared to the loop forming system, resulting in enhancement of sheet conveying efficiency.
However, as the sheet conveying speed increases, rollers contacting on a sheet become more prone to slip. The amount of slip is related to sheet surface characteristics. For example, when in double-sided recording, a sheet with an image transferred to a first side thereof is conveyed for image formation on a second side of the sheet, the amount of slip increases as the amount of toner used for image formation on the first side of the sheet increases. In this case, an image formed on the sheet is misaligned to a large degree, and hence even in the above described active registration system, it is difficult to correct for the misalignment.